Laser power supply apparatuses use high frequencies of 100 kHz to 1 MHz or so as the frequency of their inverter (high frequency inverter circuit). Semiconductor switches typified, for example, by a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) are used as switches in the converter unit and inverter unit of the laser power supply apparatuses.
MOSFETs contain a parasitic diode which occurs parasitically because of the device structure. For the parasitic diode part, it is not always possible to ensure device characteristics. In some cases, in order to prevent a current flow through the parasitic diode, a diode is connected in series to the MOSFET, and a diode different from the parasitic diode (a current-preventing diode for the parasitic diode) is further connected in antiparallel with the MOSFET for use. SBDs (Schottky barrier diodes) are often used as the diodes. With such a MOSFET, the electric current flows through the diode, the drain of the MOSFET, and the source of the MOSFET in order, and through the source of the MOSFET and the current-preventing diode in order. The high frequency inverter circuit for use in a laser power supply apparatus is used with an additional protection circuit (such as a Zener diode) for protecting the diode from a high voltage breakdown. Consequently, when a high voltage is applied to the diode, the protection circuit connected in parallel with the diode provides an electrical connection to suppress overvoltage breakdown of the diode (see patent literature 1).